From among the components in a radio communication device, the power amplifier consumes the most electric power. Thus, the improvement of power efficiency of the power amplifier will become an important task to solve. In recent years, a radio communication standard that uses an amplitude modulation system so as to improve spectrum efficiency has been becoming mainstream. Since this amplitude modulation system strictly restricts signal distortion, the power amplifier needs to be operated in a high back-off state (low input power) in which good linearity can be obtained. However, if the power amplifier is operated in the high back-off state, the power efficiency thereof will be reduced.
Thus, in recent years, polar modulation techniques such as those that maintain the linearity between input and output signals, while improving the power efficiency of power amplifiers, have been aggressively studied.
FIG. 1 shows an exemplary structure of a power amplifier based on the ET (Envelope Tracking) system as one sort of polar modulation technique.
The ET system is a technique that improves power efficiency while maintaining the linearity between input and output signals in such a manner that an RF (Radio Frequency) modulation signal containing an amplitude modulation (AM: Amplitude Modulation) component and a phase modulation (PM: Phase modulation) component is amplified, the AM component is extracted from the RF modulation signal, and the amplified RF modulation signal is amplitude-modulated with the AM component.
As shown in FIG. 1, a power amplifier based on the ET technique according to the related art is provided with polar modulator 311, RF-PA (Power Amplifier) 306, and power supply modulator 304.
Polar modulator 311 extracts an AM component from an RF modulation signal that is inputted from terminal 301 or an RF modulation signal in which a base band signal that is inputted from terminal 301 is superimposed on a carrier and the extracted AM component is outputted to power supply modulator 304 through terminal 302. In addition, polar modulator 311 outputs the RF modulation signal that is inputted from terminal 301 or RF modulation signal 308 in which the base band signal that is inputted from terminal 301 is superimposed on the carrier to RF-PA 306 through terminal 307.
FIG. 2 is a block diagram showing an exemplary structure of a polar modulator used for a power amplifier based on the ET system. FIG. 3 is a block diagram showing another exemplary structure of the polar modulator used for the power amplifier based on the ET system. Polar modulator 311 shown in FIG. 2 is a preferred example used for a structure in which the RF modulation signal is inputted from terminal 301, whereas polar modulator 311 shown in FIG. 3 is a preferred example used for a structure in which the base band signal is inputted from terminal 301.
Polar modulator 311 shown in FIG. 2 is structured with amplitude detector 130 that extracts an AM component from an RF modulation signal that is inputted from terminal 301 and outputs the extracted AM component as amplitude component signal 303 from 302 and delay compensator 131 that adjusts a delay amount of the RF modulation signal that is inputted from terminal 301 and outputs the adjusted signal from terminal 307.
The RF modulation signal that is inputted from terminal 301 is supplied not only to amplitude detector 130, but also to delay compensator 131. In polar modulator 311 shown in FIG. 2, delay compensator 131 can adjust the difference of delay times of amplitude component signal 303 that is outputted from amplitude detector 130 and RF modulation signal 308 that is outputted from terminal 307.
On the other hand, polar modulator 311 shown in FIG. 3 is structured with base band signal processing circuit 150 and quadrature modulator 152. Base band signal processing circuit 150 is structured for example with a DSP (Digital signal Processor) and a D/A (Digital to Analog) converter.
Base band signal processing circuit 150 causes the DSP to compute and extract an AM component from a base band signal that is inputted from terminal 301 through a digital process, causes the D/A converter to convert the AM component into an analog signal, and outputs the converted signal as amplitude component signal 303 from terminal 302. In addition, base band signal processing circuit 150 causes the D/A converter to convert the base band signal that is inputted from terminal 301 into an analog signal and then outputs the converted signal to quadrature modulator 152.
Quadrature modulator 152 up-converts the base band signal that is outputted from base band signal processing circuit 150 to an RF frequency and outputs the resultant signal as RF modulation signal 308 from terminal 307. Base band signal processing circuit 150 is provided with a function that can set respective output timings of amplitude component signal 303 and RF modulation signal 308.
Power supply modulator 304 amplifies amplitude component signal 303 that is outputted from polar modulator 311 and that supplies the amplified signal to power supply terminal 309 of RF-PA (Radio Frequency Power Amplifier) 306.
RF-PA 306 amplifies RF modulation signal 308 that is outputted from polar modulator 311. At this point, since amplitude component signal 305 amplified by power supply modulator 304 has been supplied to power supply terminal 309 of RF-PA 306, when the output power of RF-PA 306 is low, the voltage supplied to power supply terminal 309 also becomes low. Thus, the power supplied as a power supply to RF-PA 306 is suppressed to the minimally required power and thereby wasteful power consumption is reduced.
Besides the above-described ET system, as another sort of polar modulation techniques, the EER (Envelop Elimination and Restoration) system is also known.
The EER system is a technique that restores an original waveform of an input signal while linearly amplifying it in such a manner that an AM component is removed from an RF modulation signal containing an AM component and a PM component, so that only the remaining PM component is amplified, and the amplified PM component is amplitude-modulated with the AM component. The structure of a power amplifier based on the EER system according to the related art is shown in FIG. 4.
As shown in FIG. 4, the power amplifier based on the EER system has the same structure as the power amplifier based on the ET system shown in FIG. 1 except for the structure and operation of polar modulator 311. Thus, here, an explanation for structural components other than polar modulator 311 will be omitted. Structural components shown in FIG. 4 are denoted by similar reference numbers to those of the power amplifier shown in FIG. 1.
FIG. 5 is a block diagram showing an exemplary structure of a polar modulator used for a power amplifier based on the EER system, whereas FIG. 6 is a block diagram showing another exemplary structure of the polar modulator used for the power amplifier based on the EER system. Polar modulator 311 shown in FIG. 5 is a preferred example used for a structure in which an RF modulation signal is inputted from terminal 301, whereas polar modulator 311 shown in FIG. 6 is a preferred example used for a structure of which a base band signal is inputted from terminal 301.
Polar modulator 311 shown in FIG. 5 is structured with amplitude detector 130 that extracts an AM component from an RF modulation signal that is inputted from terminal 301 and outputs the extracted AM component as amplitude component signal 303 from terminal 302; delay compensator 131 that adjusts the delay amount of the RF modulation signal that is inputted from terminal 301 and outputs the adjusted RF modulation signal; and limiter 132 that removes the AM component from the signal that is supplied from delay compensator 131. Limiter 132 removes the AM component from the RF modulation signal that is outputted from delay compensator 131 and outputs phase component signal 313 as the remaining PM component from terminal 307. In polar modulator 311 shown in FIG. 5, delay compensator 131 can adjust the difference of delay times of amplitude component signal 303 that is outputted from amplitude detector 130 and phase component signal 313 that is outputted from terminal 307.
In contrast, polar modulator 311 shown in FIG. 6 is structured to include base band signal processing circuit 150 and VCO (Voltage Controlling Oscillator) 151. Base band signal processing circuit 150 is structured to include, for example, a DSP and a D/A converter.
Base band signal processing circuit 150 causes the DSP to compute and extract an AM component from a base band signal that is inputted from terminal 301 through a digital process, causes the built-in D/A converter to convert the AM component into an analog signal, and outputs the converted signal as amplitude component signal 303 from terminal 302. In addition, base band signal processing circuit 150 causes the DSP to compute and extract the PM component from the base band signal that is inputted from terminal 301 through a digital process, generate a control signal that causes VCO 151 to output phase component signal 313, causes the built-in D/A converter to convert the generated signal into an analog signal, and outputs the converted signal to VCO 151.
VCO 151 outputs phase component signal 313 that is up-converted into an RF frequency according to the control signal that is outputted from base band signal processing circuit 150. In addition, base band signal processing circuit 150 has a function that can set respective output timings of amplitude component signal 303 and phase component signal 313.
Like the power amplifier based on the ET system shown in FIG. 1, the power amplifier based on the EER system shown in FIG. 4 causes power supply modulator 304 to amplify amplitude component signal 303 that is outputted from polar modulator 311 and supplies the amplified signal to power supply terminal 309 of RF-PA 306.
RF-PA 306 amplifies phase component signal 313 that is outputted from polar modulator 311. At this point, since amplitude component signal 305 amplified by power supply modulator 304 has been supplied to power supply terminal 309 of RF-PA 306, when the output power of RF-PA 306 is low, the voltage supplied to power supply terminal 309 also becomes low. Thus, the power supplied as a power supply to RF-PA 306 is suppressed to the minimally required power and thereby wasteful power consumption is reduced.
In the above-described power amplifier based on ET type and EER systems, power supply modulator 304 needs to satisfy all wide band (high speed) characteristics, and wide dynamic range (high voltage operation, low noise) characteristics, and high power efficiency. However, it is difficult for power supply modulator 304 according to the related art to satisfy all these requirements.
For example, if a linear regulator is used for power supply modulator 304, although wide band (high speed) characteristics and wide dynamic range (low noise) characteristics can be accomplished, it is difficult to accomplish high power efficiency. In contrast, if a switching regular is used for power supply modulator 304, although high power efficiency can be accomplished, it is difficult to accomplish wide band (high speed) characteristics and wide dynamic range (low noise) characteristics.
In addition, since a transistor used for signal amplification has a tendency in which the operation speed is in inverse proportion to the maximum operation voltage, it is further difficult for power supply modulator 304 that is structured to include only transistors to satisfy both high voltage operation and wide band (high speed) characteristics.
Further, with respect to polar modulation techniques, Non-Patent Literature 1 has pointed out that when amplitude component signal 305 supplied to power supply terminal 309 of RF-PA 306 is low, distortion occurs in RF modulation output signal 310 that is outputted from RF-PA 306.
As countermeasures, Non-Patent Literature 1 has proposed that a lower limit value be set for amplitude component signal 305 supplied to power supply terminal 309 of RF-PA 306.
A method that can solve the problem of such polar modulation techniques is described for example in Patent Literature 1. Next, a power amplifier described in Patent Literature 1 will be explained with reference to FIG. 7(a) to (c).
FIG. 7 shows a structure and an operation of the power amplifier described in Patent Literature 1: (a) of the drawing being a block diagram showing a circuit structure; (b) of the drawing being an exemplary waveform diagram showing an input signal; (c) of the drawing being a waveform diagram showing an exemplary output signal of a power supply modulator.
In the power amplifier shown in FIG. 7(a), if the amplitude component of the input signal is lower than a threshold L denoted by a dotted line of FIG. 7(b), a direct current voltage that is a constant voltage is supplied as a power supply from VE-LOAD 205 to RF-PA 204.
If the amplitude component of the input signal is higher than threshold L, as shown in FIG. 7(c), a signal in which only the amplitude component of the input signal that exceeds the threshold L is amplified, the amplified signal will be added to the direct current voltage that is outputted from VE-LOAD 205, and the added voltage will be supplied as a power supply to RF-PA 204 from analog power valve (APV) 203.
In the power amplifier shown in FIG. 7(a), since a direct current component that occupies most of the amplitude signal supplied as a power supply to RF-PA 204 is supplied from VE-LOAD 205 structured to have a DC-DC converter having a high power efficiency, the power efficiency of the entire power amplifier can be improved. In addition, by setting a lower limit for the voltage supplied from APV 203 that amplifies the amplitude component of the input signal to RF-PA 204, APV 203 can be structured to have a circuit having a narrow dynamic range and when the power supply voltage is low, distortion that occurs in the output signal of RF-PA 204 can be reduced.
Moreover, another method that solves the problem of the above-described polar modulation techniques is described for example in Patent Literature 2. Next, a power amplifier described in Patent Literature 2 will be explained with reference to FIG. 8.
FIG. 8 is a block diagram showing a structure of the power amplifier described in Patent Literature 2.
The power amplifier shown in FIG. 8 is structured in such a manner that a low frequency component of an input signal is amplified using low pass filter (LPF) 1 and DC/DC converter 2, a high frequency component of the input signal is amplified using class B amplifier 3 and high pass filter (HPF) 4, the amplified low frequency component and high frequency component are combined, and the combined components are supplied as a power supply voltage to an RF-PA (not shown).
Since the power amplifier shown in FIG. 8 uses DC/DC converter 2 that can amplify only a low frequency component with high power efficiency and class B amplifier 3 that can amplify a high frequency component with low power efficiency, the power amplifier can amplify a signal with relatively high power efficiency and wide band (high speed).
Furthermore, another method that solves the problem of the above-described polar modulation techniques is described for example in Patent Literature 3. A power amplifier described in Patent Literature 3 will be explained with reference to FIG. 9.
FIG. 9 is a block diagram showing a structure of the power amplifier described in Patent Literature 3.
The power amplifier shown in FIG. 9 is structured with a plurality of DC-DC converters 104 through 108 and switches 110 through 114 that switch output voltages of DC-DC converters 110 through 114 and output a selected output voltage such that an optimum voltage is supplied to a collector of a transistor with which a linear regulator is provided through switches 110 through 114.
The power amplifier shown in FIG. 9 switches a direct current component of a power supply voltage that is supplied to RF-PA 124 corresponding to an amplitude value of an amplitude component signal so as to realize all wide band characteristics, wide dynamic range characteristics, and low power efficiency that cannot be realized by a circuit that uses a single power supply.
In the power amplifier shown in FIG. 7(a) (described in Patent Literature 1) from among the above-described related art, the waveform of the power supply voltage supplied to the RF-PA takes on a hard clipping shape as shown in FIG. 7(c). For example, Japanese Patent Laid-Open No. 2005-45782A has pointed out that if a voltage having such a differentially discontinuous waveform is supplied as a power supply to the RF-PA, distortion occurs in an output signal of the RF-PA. The differentially discontinuation denotes that a curve that represents a change of a power supply voltage cannot be mathematically differentiated (namely, a change ratio is discontinuous).
On the other hand, since the power amplifier shown in FIG. 8 (described in Patent Literature 2) uses a class B amplifier with relatively low power efficiency, the improvement of power efficiency of the entire power amplifier is limited. In addition, since the power amplifier described in Patent Literature 2 is provided with a circuit that separates an input signal into a low frequency component and a high frequency component and a circuit that combines the low frequency component and the high frequency component that have been amplified and thereby the circuit scale becomes large, it is difficult to miniaturize the power amplifier and reduce the cost thereof. Moreover, since the power amplifier described in Patent Literature 2 combines the low frequency component and the high frequency component that have been amplified and supplies the combined components as a power supply voltage to the RF-PA, distortion occurs in the output signal of the RF-PA due to the combination.
On the other hand, since the power amplifier shown in FIG. 9 (described in Patent Literature 3) needs to provide a power supply device that generates a plurality of direct current voltages, a plurality of switches that switch these output voltages, and a control circuit that controls the operations of the switches and thereby the circuit of the power amplifier becomes complicated and the circuit scale becomes large, it is difficult to miniaturize the power amplifier and reduce the cost thereof.
In other words, since the power amplifiers described in Patent Literature 1 and Patent Literature 2 have a problem in which a distortion occurs in an output signal caused by a circuit added to improve power efficiency and a problem in which a power consumption increase is caused by a circuit that is added to prevent the distortion from occurring in the output signal, both the improvement of power efficiency and the suppression of distortion that occurs in the output signal cannot be satisfied.
On the other hand, since the power amplifier described in Patent Literature 3 needs to additionally provide a large scale circuit that satisfies both the improvement of power efficiency and the suppression of distortion that occurs in the output signal, it is difficult to miniaturize the power amplifier and reduce the cost thereof.